1. Field of the Invention
This invention relates to the field of power supplies for portable computer systems, and more particularly, to an inverter circuit with dual converters including a synchronously switching voltage regulator and a self-resonant oscillator for supplying power to cold cathode fluorescent lamps.
2. Description of the Related Art
Liquid crystal displays with cold cathode fluorescent lamp (CCFL) back lighting are widely used laptop computer displays. Circuits for supplying power to CCFLs require a controllable alternating current power supply and a feedback loop to accurately monitor the current in the lamp in order to maintain operating stability of the circuit and to have an ability to vary the lamp brightness. Such circuits must be able to generate a high voltage to turn on the fluorescent lamp and then lower the voltage when current begins to flow in the lamp.
Inverter circuits convert unregulated DC voltage to regulated AC current and typically include a switching regulator, which are classified into different configurations or "topologies." One such topology is the single-ended inductor circuit, consisting of relatively simple circuits where a switch determines whether the voltage applied to an inductor is the input voltage, V.sub.dc, or zero. In this manner, the output voltage is a function of the average voltage applied to the inductor. The switch may be implemented using various electronic components, for example, a power transistor, coupled either in series or parallel with the load. The regulator controls the turning ON and turning OFF of the switch in order to regulate the flow of power to the load. The switching regulator employs inductive energy storage elements to convert the switched current pulses to a steady load current. Power in a switching regulator is thus transmitted across the switch in discrete current pulses.
In order to generate a stream of current pulses, switching regulators typically include control circuitry to turn the switch on and off. The switch duty cycle, which controls the flow of power to the load, can be varied by a variety of methods. For example, the duty cycle can be varied by either (1) fixing the pulse stream frequency and varying the ON or OFF time of each pulse, or (2) fixing the ON or OFF time of each pulse and varying the pulse stream frequency. Which ever method is used to control the duty cycle, the switch in switching regulators is either OFF, where no power is dissipated by the switch, or ON in a low impedance state, where a small amount of power is dissipated by the switch. This generally results in fairly efficient operation with regard to the average amount of power dissipated.
One method that has been utilized to improve operational efficiency of voltage regulators employs synchronous rectification. In synchronous rectification, a pair of switches, which are connected in series between the input voltage and ground, are synchronized so that either the input voltage or ground is applied to the input of an inductor. The synchronous control of the switches provides improved efficiency compared to traditional circuits which employed a switch and a diode.
In the prior art, the output of the voltage regulator is used to drive one or more lamps for illuminating the display. A transformer converts the input from the voltage regulator to a current signal having the frequency and magnitude required to drive the lamp. It is desirable to reduce the size of the transformers currently used in display assemblies while providing the same or greater power to the lamp. It is also desirable to provide an inverter circuit that dissipates less heat, which can cause spotting, discoloration, and poor color purity of the display.
Energy loss due to various parasitic paths typically occurs in display assemblies. For example, energy is lost in the wire that connects the secondary winding of the transformer to the first end of lamp, while parasitic capacitance losses cause lost energy in the lamp itself. Incremental energy losses accumulate over the length of the lamp starting at the grounded end, reaching a maximum value at the non-grounded end. Another source of parasitic loss is due to electrical interference with light reflector in a display assembly, which is typically constructed of metallic material. It is therefore desirable to provide an inverter circuit that provides the same or greater power to one or more lamps while reducing the parasitic energy loss.
The inverter circuit is typically mounted on one of the sides of the display panel, thereby adding width to the panel assembly. In the past, the keyboard in a laptop computer was usually wider than the display, however, as display size increases beyond the size of the keyboard in more recent laptop computers, it is desirable to move the inverter circuit from the side of the display to another location to avoid increasing the width of the display housing. The inverter circuit may be located behind the display panel, however, it is desirable to avoid increasing the depth of the panel assembly. Additionally, the repositioned inverter circuit must not cause radio frequency interference or electromagnetic interference with the display. In the prior art, some display assemblies include thermal and radiation shields, however, these shields add cost, increase parasitic losses, induce eddy currents, and increase the amount of material that must be included with the display assembly.
As notebook computer displays increase in physical size, the power required to illuminate the display also increases. To improve customer satisfaction with portable battery-operated equipment, in particular notebook computers, it is desired to provide equipment that is highly energy efficient and as lightweight and compact as possible. Further, computer display panels currently incorporate numerous components that must be integrated by equipment manufacturers. It is desirable to integrate the functionality of the components into one standardized assembly to reduce manufacturing cost and complexity.